Arrangements of this kind are designed so that the exhaust gases from each bank of cylinders are passed via separate exhaust pipes to catalytic converters located on the same side of the vehicle. However, this arrangement has the disadvantage that the converter can only be mounted either close to the engine or distant from it, so that it is not possible to provide a short additional heating-up path. A heating-up path would necessarily have about the same length as the main pipe train.
In the earlier-filed German patent application DE-196 53 408.9-13 it has already been proposed to lead the exhaust gases from the cylinders on each one side via an exhaust manifold and an exhaust pipe to the respective other side of the engine and to a catalytic converter also located on said other side, with a control valve unit being incorporated in each exhaust pipe by which the exhaust pipe can be shut off, in each case a respective by-pass pipe of markedly smaller cross-section than the exhaust pipes being provided between the exhaust pipe of one bank of cylinders and the exhaust pipe of the other bank of cylinders, said by-pass pipe connecting a region of the exhaust pipes near to the exhaust manifold of one bank of cylinders to a region near where the exhaust pipe opens into the catalytic converter, located on the same side of the vehicle, for the other bank of cylinders.
However, with this arrangement it can happen that as a result of the exothermic reaction of the three-way converter used, the NOx-trap which is also located in this region is undesirably heated. it will be appreciated! It is understood that NOx-traps cannot stand very high temperatures.